An inspection, be it optical or electron beam, is typically performed at many different times during the fabrication cycle of a modern integrated circuit. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
The inspection of integrated circuits is typically accomplished using a comparison process, where information from the image of a test die is compared to information from the image of a reference die. These two information sets are subtracted, one from the other, to yield a difference set. Because the test die and the reference die are assumed to be identical, any non-zero values in the difference set are assumed to be the result of defects on either the test die or the reference die.
In some inspection routines, the reference die is an absolute reference that is known to be non-defective. In this case, any non-zero values are properly attributable to defects in the test die. In other inspection routines the reference die is a floating reference, such as another die on the same substrate as the test die. In this case, the non-zero values in the difference set might be attributable to either the test die or the reference die, and further investigation is required.
However, in some cases the floating reference die and the test die both have a defect in the same location on each die. When this occurs, a subtraction of the information from the two images yields a zero value in the position within the difference set that indicates the location of the two defects. However, zero values are interpreted as no defect, rather than a matching defect on both of the dice.
What is needed, therefore, is an inspection system that overcomes problems such as those described above, at least in part.